The present invention relates to a method of controlling a power train such that a slip is prevented at start-up and acceleration of an automotive vehicle on a slippery road.
According to a conventional method of controlling a power train so as to prevent slip from occurring, as disclosed in Laid-open Japanese Patent Application No. 58-38347, slip is detected based on a difference between an average between revolution speeds of two front wheels and that between revolution speeds of two rear wheels (driving wheels), and the supply of fuel to an engine, which is a power source of the power train, is discontinued when the slip is detected in order to prevent the slip from occurring.
Referring to FIG. 8, there is shown how a driving force of a driving wheel varies as a function of a variation in a slip rate S of the driving wheel for different road conditions including a dry road, a wet road and a snow-covered road. The slip rate S is expressed as EQU S=(PSD-GSV)/PSD
where:
PSD is an abbreviation for the peripheral speed of a driving wheel, PA1 GSV is an abbreviation for the ground speed of a vehicle. PA1 According to the present invention there is provided a method of controlling an automotive vehicle having a driving wheel driven by a power train and a non-driving wheel, the power train including an engine and a transmission, the method comprising the steps of: PA1 detecting a peripheral speed of the driving wheel and generating a first peripheral speed indicative signal indicative of said peripheral speed of the driving wheel; PA1 detecting a peripheral speed of the non-driving wheel and generating a second peripheral speed indicative signal indicative of said peripheral speed of the non-driving wheel; PA1 determining a ratio of a difference between said first and second peripheral speed indicative signals to said first peripheral speed indicative signal and generating a slip rate indicative signal indicative of the value of said determined ratio; PA1 determining from the slip rate indicative signal a deviation of the value of said determined ratio from a predetermined target slip rate value and generating a deviation indicative signal indicative of said determined deviation; and PA1 controlling the power train in response to said deviation indicative signal such that an output torque produced by the power train is varied in a direction as to decrease said deviation indicative signal toward zero whereby said indicative value of said slip rate indicative signal is kept at said predetermined target slip rate value.
A driving force which is applicable to each driving wheel by a power train differs and is determined by the road surface condition and a slip rate S. That is, the driving force decreases as the road surface condition which the vehicle runs on changes from dry to wet and then to snow, and the driving force peaks and becomes maximum at a predetermined slip rate S1 for all of the road conditions and becomes smaller and smaller as the deviation increases from this slip rate S1.
The conventional method poses a problem which is attributed to the face that slip is prevented by causing a reduction in an output torque of an engine upon detecting slip when a difference between an average of the revolution speeds of the two front wheels (ground speed of vehicle or GSV) and that of the revolution speeds of the two rear driving wheels (peripheral speed of driving wheel or PSD) exceeds a predetermined value. In other words, what is effected by this conventional controlling method is to cause the slip rate S to decrease only when the peripheral speed of the driving wheels is higher than the average of the revolution speeds of the front wheels (non-driving wheels). Again referring to FIG. 8, this conventional control does not allow the slip rate to be increased for example when the slip rate is smaller than the predetermined slip rate S1, where the driving force becomes a maximum. The problem is therefore that when the vehicle has to run with slip rate falling in a range smaller than the predetermined slip rate S1, the vehicle is forced to be driven with an insufficient driving force, thus failing to generate a sufficient driving force for quick starting of the vehicle at start-up, failing to give a sufficient degree of acceleration, and failing to make full use of the maximum output which can be generated by the power train.